vlz,
ksh dynamically allocates space for variable names and does not impose a limit on the length of a name. There is a limit on the maximum length of an input line and this will indirectly impose a limit on variable name length. Your version of ksh should guarantee that it can handle a command line of at least LINE_MAX length. Your value for LINE_MAX may be on a limits man page.
However, good programming technique will dictate much smaller variable sizes.